1. Field of the Invention
The present invention relates to a semiconductor integrated circuit device incorporating a voltage detecting circuit; in particular it relates to a semiconductor integrated circuit device incorporating a voltage detecting circuit that can detect the drop in operating power supply voltage.
2. Description of the Prior Art
To prevent malfunction of a semiconductor integrated circuit device (hereafter, referred to as LSI) from occurring when voltage rises at the time of energization start-up, or when the power supply voltage drops during operation, a method that is well-known involves the monitoring of the power supply voltage by a power supply voltage detecting circuit provided and the transmission of a reset signal to halt the operation of the LSI when the power supply voltage becomes lower than that specified.
For example, in Japanese Patent Application Laid-Open No. Hei 2-228064 (hereafter, referred to as the well-known example), an example of a voltage detecting circuit that can detect a drop in operating power supply voltage is disclosed. FIG. 1 is a block diagram showing the configuration of a detecting circuit that is disclosed in this well-known example.
Referring to FIG. 1, this conventional detecting circuit 550, which comprises step-up circuit 513, reference voltage generation circuit 515, voltage dividing circuit 517, comparator 519, and interface circuit 521, is mounted on a single semiconductor chip comprising both bipolar elements and CMOS elements.
Step-up circuit 513 with a configuration equivalent to that of a well-known DCxe2x80x94DC converter converts the power supply voltage Vcc, which is supplied to bipolar elements, (e.g., 1.5 V), which is supplied to terminal 511 to alternating voltage, steps it up, and then reconverts it to direct-current voltage VDD (e.g., 3.6 V) which can drive a CMOS circuit, and supplies voltage VDD to the CMOS logic circuit (not shown in the drawing) via terminal 514.
Reference voltage generation means 155 consists of constant-current source Io, resistance R11, and PN junction diode Q1, generating reference first voltage VN at connection point P. VN can be set to a desired value by changing the resistance R11 value. Diode Q1 is connected between the base and the emitter of NPN transistor Q2, transistor Q2 thus operates as a current source.
Voltage dividing means 517, consisting of resistances R12 and R13, provides second voltage Vd at connection point S by dividing stepped-up voltage VDD. In other words, Vd=VDDxc3x97(R13/(R12+R13)).
Comparator 519 is a differential amplifier that has PNP transistors Q5 and Q6 as load elements, NPN transistors Q3 and Q4 as driving elements, and NPN transistor Q2 as a current source element. One input terminal IN1 is connected to connection point S, whereas the other input terminal IN2 is connected to connection point P. Output terminal OUT1 is connected to interface circuit 521 via terminal 520.
Interface circuit 521 consists of resistance R14 and NPN transistor Q7. When voltage V01 of comparator output terminal 520 is high level, voltage V02 of detection signal transmission terminal 522 (connection point T) is equal to saturation voltage VCE (sat) between the collector and the emitter of transistor Q7; when voltage V02 is low level, voltage V02 is approximately equal to VDD.
FIG. 2 is a schematic for describing an overview of the operation of detecting circuit 550, wherein the horizontal axis is time t, the vertical axis shows reference voltage (first voltage) VN, stepped-up voltage VDD, and second voltage (divided voltage) Vd. When time t=0, standby mode is cancelled initiating the operation of step-up circuit 513. Curves 541, 542, and 543 in FIG. 2 indicate the relationships between each of VN, VDD, and VD and time t, respectively. VDDX is a preferred, to-be-detected stepped-up voltage that has been specified in advance. Circuit constants for reference voltage generation means 515 and voltage dividing means 517 are preset to values that satisfy the relationship of divided voltage Vdx of stepped-up voltage VDDX being equal to reference voltage VN. Therefore, when VDD less than VDDX, Vd less than VN and accordingly voltage information V01 of output terminal 520 of the comparator is high level; whereas when VDD greater than VDDX, Vd greater than VN, and consequently voltage information V01, is low level. Accordingly, the to-be-detected, stepped-up voltage VDDX can be detected.
In the conventional detecting circuit 550 described above, since reference voltage generation means 515 and comparator 519, which are configured using bipolar elements that can be activated by a power supply that is externally supplied to bipolar elements of a low voltage to detect the stepped-up voltage, the detection of low voltage down to a certain point becomes sufficiently possible.
However, in the conventional detecting circuit 550, both reference voltage generation means 515 and comparator 519 are driven by power supply voltage VCC, which is supplied for bipolar circuits. This configuration develops a problem where the stability of reference voltage VN that is generated by voltage generation means 515 against temperature change is degraded, particularly at the supplied voltage 1 V or lower; and accordingly, the fluctuation in the reference voltage becomes larger. This makes it difficult to generate, for example, a power-on/reset signal accurately in such a case of low voltage.
Accordingly, the objective of the present invention is to provide an LSI, which comprises a voltage detecting circuit that can detect the power supply voltage accurately and surely, even if the power supply voltage supplied externally falls to 1 V or less.
Therefore, an LSI, according to the present invention, which is driven by the first voltage as a power supply supplied to an external power supply terminal to drive internal circuits with desired functions, such as a CPU or peripheral circuits, comprises at least: a step-up circuit, which is driven by the first voltage supplied to an external power supply terminal as a power supply and which steps up the first voltage at a predetermined ratio into a second voltage and outputs it; a voltage detecting circuit, which is driven by the second voltage as a power supply and which compares a predetermined reference voltage to a divided voltage given by dividing the second voltage and outputs a first comparison result signal; and a level shift circuit, which is driven using the first voltage as a power supply and which changes the level of the first comparison result signal to outputs a second comparison result signal.
At this time, the voltage detecting circuit may comprise at least a reference voltage generating circuit, which generates a reference voltage; a dividing circuit which divides the second voltage into a divided voltage; and a comparison circuit which inputs the reference voltage and the divided voltage and outputs a first comparison result signal.
Furthermore, the level shift circuit may be configured so that the ratio of the second voltage to the first voltage is equal to that of the first comparison result signal level to the second comparison result signal level.
It is noted that the second comparison result signal may be used as a reset signal for resetting an internal circuit when the divided voltage is lower than the reference voltage.
Furthermore, it is preferable that the reference voltage generating circuit is structured with a bandgap circuit.
Features of the LSI according to the present invention are: including a voltage detecting circuit and other internal circuits; stepping up a detected voltage from the voltage detecting circuit onto an appropriate voltage by the step-up circuit; and driving the voltage detecting circuit by the stepped-up voltage, which is provided by the step-up circuit.